After checking the content of the project folder, it appears I might have found a solution to retrieve at least 80% of the project.
The bug actually only erased 2 Sheets and layout XML files while the rest of images and other XML were fine in the caproj folder.
So I just replaced the missing XML by their old version from 1 month ago I had in another caproj folder and I also replaced the caproj which was corrupted by the 1 month old version
Of course, a lot of objects were missing and the caproj had a lot of missing layouts and event sheets. However each time I opened the caproj, Construct 2 displayed a warning message with the file and line having the missing object and I've been able to add manually the XML node missing each time. Also put the missing object into the correct family I could remember.
Overall, I had about 40 missing objects and using this method allowed me to keep the objects, conditions and events from the latest version. Only 2 layout got switched back to their 1 month old version but it's better than losing everything and they're not that big.
The only thing I did not figure out in the XML files is the SID attribute in each nodes. Since I duplicated similar object to fill the blanks, I have many identical SID in the new objects. I thought it might be a problem but after successfully launching the caproj while having most of the recent version content and also successfully launching the game in the browser, I'm not sure what this SID is used for. Is it just a random unique ID created by the software or is there a real use to it ?
I know using such a method must sound horrible but so far, it seemed to work fine. As long as it works, I don't mind using it.